This invention relates to an interior permanent magnet synchronous motor wherein a rotor core has a plurality of permanent magnets incorporated or embedded therein and includes magnetic salient pole sections defined between each two adjacent permanent magnets, and more particularly to a permanent magnet-equipped synchronous motor utilizing both reluctance generated due to the salient pole sections of the rotor core and torque by the permanent magnets.
One of conventional synchronous motors wherein a core between magnetic poles of permanent magnets is provided with magnetic salient pole sections is disclosed in Japanese Patent Application Laid-Open Publication No. 205499/1996. The synchronous motor is constructed in such a manner that rotation of a rotor is limited to only one direction, to thereby displace the silent pole sections, resulting in restraining generation of torque pulsation.
Also in Japanese Patent Application Laid-Open Publication No. 256455/1996 a technology in which generation of torque pulsation is restrained by changing the width of magnetic poles of magnetic salient pole sections of reluctance synchronous motor or by displacing a part of pairs of magnetic salient pole sections thereof in a peripheral direction is disclosed.
Another conventional synchronous motor having permanent magnets incorporated therein is disclosed in Japanese Patent Application Laid-Open Publication No. 18328/1999. The conventional synchronous motor disclosed is so constructed that a width of a core between magnetic poles of permanent magnets is set so as to establish relationship represented by the following expression, to thereby restrain generation of cogging torque:
xcex8minxe2x89xa6xcex8xe2x89xa6xcex8max 
In the conventional interior permanent magnet synchronous motor, the open angle xcex8 of the rotor core between permanent magnet poles is defined to be within a range of xcex8minxe2x89xa6xcex8xe2x89xa6xcex8 max determined depending on the number of teeth, a configuration thereof and a size thereof. However, a timing at which torque is generated between the magnetic poles of the permanent magnets is varied depending on xe2x80x9cthe number of slots per pole and per phasexe2x80x9d q of a stator, so that the synchronous motor fails to satisfactorily restrain cogging torque and torque pulsation.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a permanent magnet-equipped synchronous motor which is capable of sufficiently restraining both cogging torque and torque pulsation during feeding of electricity thereto.
In accordance with the present invention, a permanent magnet-equipped synchronous motor is provided. The permanent magnet-equipped synchronous motor includes a stator including a stator core provided with a plurality of magnetic pole sections having windings of at least one phase wound thereon, as well as a rotor having p pole pairs (p: a positive integer of 1 or more) The rotor includes a shaft and a rotor core fixed on the shaft. The rotor core has 2p (a plural number) permanent magnets incorporated therein in a manner to be spaced from each other at intervals in a peripheral direction thereof. The 2p permanent magnets each constitute a permanent magnet magnetic pole section formed on an outer periphery of the rotor core. The rotor is formed with 2p magnetic salient pole sections arranged so as to interpose the permanent magnet magnetic pole section therebetween. It is to be noted that herein xe2x80x9cone permanent magnetxe2x80x9d means not only one permanent magnet in physical sense, but also such xe2x80x9cone permanent magnetxe2x80x9d as comprises a plural permanent magnets and yet functions as one permanent magnet magnetically.
According to the present invention the 2p permanent magnet magnetic pole sections comprise two groups (a first and a second groups) of permanent magnet magnetic pole sections. Each permanent magnet magnetic pole section of the first group is arranged to be spaced at equal intervals in the peripheral direction interposing one permanent magnet magnetic pole section of the second group between each two adjacent permanent magnet magnetic pole sections. Similarly, each permanent magnet magnetic pole section of the second group is also arranged to be spaced at equal intervals in the peripheral direction interposing one permanent magnet magnetic pole section of the first group mentioned above between each two adjacent permanent magnet magnetic pole sections. In other words each permanent magnet magnetic pole section of the two groups is arranged to appear alternately in the peripheral direction.
Also the 2p (p: a positive integer of one or more) magnetic salient pole sections comprise two groups (a first and a second groups) of magnetic salient pole sections. Each magnetic salient pole section of the first group is arranged to be spaced at equal intervals in the peripheral direction interposing one magnetic salient section of the second group between each two adjacent magnetic salient pole sections. Similarly, each magnetic salient pole section of the second group is also arranged to be spaced at equal intervals in the peripheral direction interposing one magnetic salient pole section of the first group mentioned above between each two adjacent permanent magnet pole sections. In other words magnetic salient pole sections of the two groups are arranged to appear alternately in the peripheral direction of the rotor core. In this instance an open angle of each of the p magnetic salient pole sections of the first group, xcex11, is set to be smaller than an open angle of each of the p magnetic salient pole sections of the second group, xcex12. Further the open angles xcex11 and xcex12 are set to satisfy the following expression.
xcex12xe2x88x92xcex11≈2xcex2xe2x88x92(2nxe2x88x921)xcfx84s 
wherein n is a natural number, xcex2 is an angle defined between two salient pole section virtual center lines, CL1 and CL2. xcfx84s is a slot pitch of the stator core (denominated in rad). The outer peripheral surface sections of the permanent magnet pole sections of the rotor core may have a contour formed into an arcuate or elliptic configuration.
When the open angle xcex11 of the p magnetic salient pole sections of the first group and the open angle xcex12 of the p magnetic salient pole sections of the second group are defined as mentioned above, torque pulsation can be restrained and torque ripple can be diminished greatly as compared with the case in which the open angle of the 2p magnetic salient pole sections (each magnetic salient pole section of both first and second group) is set at equal value.
In this instance the curvature radius R1 of the magnetic pole surface of the p magnetic salient pole sections of the first group is set to be smaller than the curvature radius R2 of the magnetic pole surface of the p magnetic salient pole sections of the second group. Such arrangement permits torque ripple to be diminished as compared with the case in which the curvature radii R1 and R2 are set at the same value. In order to increase torque, on the other hand, the curvature radii R1 and R2 each are preferably set at a larger value than the curvature radii of the end portions of the magnetic pole surfaces of adjacent permanent magnet magnetic pole sections, and yet to satisfy the condition, R1 less than R2. However, in order to diminish torque ripple further, at the sacrifice of the torque strength, the curvature radii R1 and R2 each may, of course, be set at a smaller value than the curvature radii of the end portions of the magnetic pole surfaces of adjacent permanent magnet magnetic pole sections.
In this instance the shapes of the 2p permanent magnet magnetic pole sections and the 2p magnetic salient pole sections may preferably be determined so that the contour of the outer peripheral surface sections of the rotor core formed with two adjacent permanent magnet magnetic pole sections and a magnetic salient pole section interposed therebetween may have symmetrical shapes about the salient pole section virtual center lines (CL1, CL2) and yet so that the contour of the outer peripheral surface sections corresponding to the angle of 360xc2x0/p about the center of the shaft of the rotor each may be formed into an identical shape, thus resulting in the rotor core having p identical shapes in the outer periphery thereof. Such arrangement prevents electrical voltage unbalance or eccentric force against rotor from being generated because magnetic balance is obtained in the peripheral direction, even if open angles of magnetic salient pole sections are set at different values.
When magnetic pole surfaces of permanent magnet magnetic pole sections are formed into arcuate or elliptic shape, each of magnetic pole surfaces of permanent magnet magnetic pole sections of the rotor and each of magnetic pole surfaces of a plurality of magnetic poles of the stator core may preferably be arranged so as to have a gap defined therebetween and having a dimension xcex4d which satisfies the following expression:
xcex4d=xcex4d0/cos[p(xcex8mxe2x88x92xcex8dm)]
wherein xcex4d0 is the minimum value of the dimension of the gap, xcex8m is an angle defined from the virtual center line CL3 which extends in the center of the two salient pole section virtual center lines CL1 and CL2 toward the side of the magnetic salient pole section having the open angle xcex11. xcex8dm is an angle between the virtual line PL3 which extends from the center of the shaft through a position where the dimension of the gap has the minimum value and the virtual center line CL3.
In the above expression, when the value of xcex8dm is set at 0 (xcex8dm=0xc2x0), the gap formed will constitute a general gap called xe2x80x9ccosec gapxe2x80x9d. Such a gap configuration permits, irrespective of the direction of the rotation of the motor, a distribution of density of a magnetic flux from the permanent magnets in the gap to approach a sine wave, to thereby restrain cogging torque.
In this instance, the value of xcex8dm which permits the value of cogging torque to be minimum is determined by the expression, xcex8dm≈("psgr"2xe2x88x92"psgr"1)/2. Angles "psgr"1 and "psgr"2 will be described in the following. However, when the distribution of density of a magnetic flux from the permanent magnets in the gap deviates greatly from the sine wave, the minimum value of the cogging torque exists within a range of
(1/6)xc3x97Xxc3x97xcfx84sxe2x89xa6xcex8dmxe2x89xa6(1/2)xc3x97Xxc3x97xcfx84s 
wherein X is a natural number which makes xcex8dm most approach the value of ("psgr"2xe2x88x92"psgr"1)/2 when the following expression is almost satisfied:
xcex8dm≈("psgr"2xe2x88x92"psgr"1)/2≈(1/4)xc3x97Xxc3x97xcfx84s 
In addition to satisfying the above condition, the following expressions should be satisfied also while "psgr"1 is an angle defined between the virtual center line CL3 and the virtual line PL1, which is one of the two virtual lines PL1 and PL2 which extend from the center of the shaft through both ends of each of the magnetic pole sections and yet the virtual line on the side of the magnetic salient pole sections having an open angle xcex12, and "psgr"2 is an angle defined between the virtual center line CL3 and the virtual line PL2, which is the other of the two virtual lines PL1 and PL2 and yet the virtual line on the side of the magnetic salient pole sections having an open angle xcex11:
"psgr"2 greater than "psgr"1 
"psgr"2xe2x88x92"psgr"1≈0.5(2mxe2x88x921)xcfx84sxe2x88x92(180xc2x0/p) 
"psgr"2+"psgr"1≈uxc2x7xcfx84s 
xcex11+xcex12xe2x89xa6(360xc2x0/p)xe2x88x922("psgr"2+"psgr"1) 
wherein m and u are arbitrary natural numbers. When such arrangement as above is satisfied, not only cogging torque can be diminished to the minimum value but also torque ripple can be restrained.
Moreover in a motor whose size of the gap xcex4d as mentioned above does not constitute a so-called cosec gap, it is also possible to diminish cogging torque and torque ripple when the relations between the angles "psgr"2 and "psgr"1 and open angles xcex11 and xcex12 are established as mentioned above.
Furthermore when xcex11, xcex12, "psgr"2 and "psgr"1 are set at such values as to satisfy the following expressions, both cogging torque and torque ripple can be arranged to approach the minimum values.
(180xc2x0/2p)+(xcex11/2)xe2x88x92"psgr"2≈(1/4)(2v1xe2x88x921)xcfx84s 
(180xc2x0/2p)+(xcex12/2)xe2x88x92"psgr"1≈(1/4)(2v2xe2x88x921)xcfx84s 
wherein v1 and v2 are arbitrary natural numbers.
In order to form a first and a second non-magnetic sections with recesses at both ends in the peripheral direction of the permanent magnets of the rotor core, while the first non-magnetic section is arranged on the side of the magnetic salient pole section having an open angle xcex11 and the second non-magnetic section is arranged on the side of the magnetic salient pole section having an open angle xcex12, the shape of the non-magnetic sections may preferably be arranged in such a manner that the area of the cross section of the first and the second non-magnetic sections are the same or the area of the cross section of the second non-magnetic section is larger than the area of the cross section of the first non-magnetic section. Such arrangement permits leakage of magnetic flux from permanent magnets (short circuit of magnetic flux) to be restrained as well as demagnetization to be prevented. However, according to the present invention, the open angle xcex11 and the open angle xcex12 have different values, thus the propensity for leakage of magnetic flux from permanent magnets and propensity for demagnetization are different at both ends
in the peripheral direction of the permanent magnets. Namely, in case of xcex11 less than xcex12, amount of leakage of the magnetic flux from permanent magnets is larger at the end in the peripheral direction on the side of the magnetic salient pole section having the open angle xcex12 than at the end in the peripheral direction on the side of the magnetic salient pole section having the open angle xcex11. This leads to more demagnetization of the permanent magnets at the end in the peripheral direction on the side of the magnetic salient pole section having an open angle of xcex12 than at the end in the peripheral direction on the side of the magnetic salient pole section having an open angle of xcex11. From the above viewpoints, the area of the cross section of the second non-magnetic section is arranged to be larger than that of the first non-magnetic section in order to positively restrain the leakage of magnetic flux from permanent magnets from the end portion on the side of the magnetic salient pole section having an open angle of xcex12 and the demagnetization of the permanent magnets at the end portion thereof.